contrail n : an artificial cloud created by an
aircraft; caused either by condensation due to the reduction in air
pressure above the wing surface or by water vapor in the engine
exhaust [syn: condensation
trail]

Anagrams

Contrails or vapour trails are condensation trails and
artificial cirrus
clouds made by the exhaust of aircraft engines or wingtip
vortices which precipitate a stream of tiny ice crystals in moist, frigid upper
air. Being composed of
water, the visible white streams are not air
pollution. However, vapour trails or contrails generated by
engine exhaust are inevitably linked with typical fuel combustion
pollutants.

Condensation from engine exhaust

The main products of
hydrocarbon fuel combustion are carbon dioxide
and water vapor. At high altitudes this water vapor emerges into a
cold environment, and the local increase in water vapor can push
the water content of the air past saturation
point. The vapor then condenses into tiny water droplets and/or
deposits into ice. These millions of tiny water droplets and/or ice
crystals form the vapour trail or contrails. The energy drop (and
therefore, time and distance) the vapor needs to condense accounts
for the contrail forming some way behind the aircraft's engines.
The majority of the cloud content comes from water trapped in the
surrounding air. At high altitudes, supercooled water vapor
requires a trigger to encourage deposition or condensation. The
exhaust particles in the aircraft's exhaust act as this trigger,
causing the trapped vapor to rapidly turn to ice crystals. Exhaust
vapour trails or contrails usually occur at above 8000 metres
(26,000 feet). where the temperature is below -40°C (-40°F).

Condensation from decreases in pressure

The wings of an
aircraft cause a drop in air pressure in the vicinity of the wing.
This brings with it a drop in temperature, which can cause water to
condense out of the air and form a vapour trail or contrail. This
effect (the Prandtl-Glauert
singularity), is more common on humid days, and can be seen on
fighter jets performing high energy maneuvers, during shuttle
launches, on the expanding surface "bubble" of nuclear explosions,
or on airliners during takeoff and landing. Additionally, the area
around a turbo-fan intake will be at a lower pressure than the
surrounding air, and may result in a condensation fog forming there
during high power settings.

These effects are compounded with the other
explanation of contrails, which is the water vapor produced by the
combustion of jet fuel. High altitude contrails are seen directly
behind the one or two jets, and with aircraft with four jets, such
as the Boeing 747, there are four contrails. The vapor trails
caused by the first mentioned effect are usually seen at low
altitude where the ambient humidity is higher, and they follow the
wings rather that the jet engines.

Vapor trails or contrails and climate

Vapor trails or
contrails, by affecting the Earth's radiation balance, act as a
radiative
forcing. Studies have found that vapour trails or contrails
trap outgoing longwave
radiation emitted by the Earth and atmosphere (positive
radiative forcing) at a greater rate than they reflect incoming
solar
radiation (negative radiative forcing). Therefore, the overall
net effect of contrails is positive, i.e. a warming. However, the
effect varies daily and annually, and overall the magnitude of the
forcing is not well known: globally (for 1992 air traffic
conditions), values range from 3.5 mW/m² to 17 mW/m². Other studies
have determined that night flights are most responsible for the
warming effect: while accounting for only 25% of daily air traffic,
they contribute 60 to 80% of contrail radiative forcing. Similarly,
winter flights account for only 22% of annual air traffic, but
contribute half of the annual mean radiative forcing.

September 11, 2001 climate impact study

The grounding of
planes for three days in the United States after
September 11, 2001 provided a rare opportunity for scientists
to study the effects of contrails on climate forcing. Measurements
showed that without contrails, the local diurnal temperature range
(difference of day and night temperatures) was about 1 degree
Celsius higher than immediately before; however, it has also been
suggested that this was due to unusually clear weather during the
period.

Distrails

Where an aircraft passes through a cloud, it can
clear a path through it; this is known as a distrail. Because the
plane's contrail is not yet visible, (because of its height,
contrails usually form at 22,000-28,000 feet, depending on the
temperature and other factors) the cloud looks like a tunnel seen
horizontally and vertically, assuming the cloud is very thin, looks
like it has been divided.